1. Field of the Invention
The present invention relates to a polymer electrolyte and a lithium battery using the same, and more particularly, to a polymer electrolyte with extended cycle life, improved safety, and reduced swelling by using a multifunctional isocyanurate monomer, and a lithium battery using the polymer electrolyte.
2. Description of the Related Art
The recent popularization of portable electronic information devices such as notebook computers and camcoders and of wireless communications devices, such as mobile phones, has increased the need for small, lightweight rechargeable secondary batteries that can be used as power supplies for such portable devices.
The most widely used secondary batteries developed to date include lithium cadmium batteries, nickel-hydride batteries, and lithium batteries. Among the cited kinds of secondary batteries, lithium batteries attract more attention as a possible next-generation power supply due to having an extended cycle life and a high capacity.
In a liquid electrolyte lithium secondary battery containing a low boiling point organic solvent for improved low-temperature performance, the electrode assembly or the pouch packaging the electrode assembly may swell when left under high temperature conditions. As a result, the high-temperature reliability and the safety of the battery degrade.
To solve this problem, methods using polymer solid electrolytes have been suggested. The use of solid polymer electrolytes reduces the likelihood of electrolyte leaking and improves battery safety, as compared with the case of using liquid electrolytes.
However, when such a polymer solid electrolyte is used, the ion conductivity becomes lower than the case of using liquid electrolytes. Therefore, there is an urgent need for the development of polymer solid electrolytes with high ionic conductivity and improved electrochemical stability in order for the polymer solid electrolytes to be applied practically to lithium secondary batteries.
Linear polymers or cross-linked polymers derived by the polymerization of alkylene oxides such as ethylene oxide and propylene oxide are used mostly as ionic conductive polymers for the polymer solid electrolyte. However, in a lithium battery using a polymer synthesized from such kinds of monomers as a matrix, the cross-linked network structure of the polymer matrix and strong interactions between lithium ions and alkylene oxide residue in the matrix lower the mobility of the lithium ions, thus resulting in degraded high rate discharge, low-temperature discharge, and reduced cycle life characteristics.
Such drawbacks can be improved by increasing the amount of organic solvent used above a predetermined level. However, the increased amount of organic solvent hinders the formation of a cross-linked polymer matrix and causes the liquid electrolyte to leak from the gel matrix. As a result, the electrode assembly or the pouch packaging the electrode assembly swells significantly at high temperature. Therefore, various performance improvements are needed for such polymer electrolyte lithium batteries using a poly(alkylene oxide) polymer.